Djokovic reportedly bought the annual output of Pule from the world's sole producer, a donkey farm 50 miles west of the Serbian capital Belgrade. The Serbian farm also produces donkey soap and bottled donkey milk, which is said to contain 60 times more vitamin C than cow’s milk, according to the Daily Mail.

Donkey milk is said to be very healthy for humans as it has anti-allergen properties, contains only one per cent milk fat, and is drunk only fresh because precious ingredients get lost if boiled, reports the Telegraph.